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Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site.
Nickel isotope fractionation during laterite Ni ore smelting and refining: implications for tracing the sources of Ni in
smelter-affected soils
Ratié G. 1, 2, *, Quantin C. 1, Jouvin D. 1, Calmels D. 1, Ettler V. 2, Sivry Y. 3, Cruz Vieira L. 2, Ponzevera Emmanuel 5, Garnier J. 2
1 UMR 8148 GEOPS, Université Paris Sud - CNRS, 91405 Cedex, France 2 UnB, IG/GMP-ICC Centro, Campus Universitario Darcy Ribeiro, 70910-970, Brasilia-DF, Brazil / Laboratoire Mixte International, LMI OCE « Observatoire des changements Environnementaux », Institut de Recherche pour le Développement / University of Brasilia, Campus Darcy Ribeiro, Brasilia, Brazil 3 Institute of Geochemistry, Mineralogy and Mineral Resources, Charles University in Prague, Albertov 6, 12843 Prague 2, Czech Republic 4 Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Univ. Paris Diderot, UMR 7154 CNRS, F-75005 Paris, France 5 IFREMER, Centre de Brest, Unité Géosciences Marines, 29280, Plouzané, France
Abstract : Nickel isotope ratios were measured in ores, fly ash, slags and FeNi samples from two metallurgical plants located in the Goiás State, Brazil (Barro Alto, Niquelândia). This allowed investigating the mass-dependent fractionation of Ni isotopes during the Ni-laterite ore smelting and refining. Feeding material exhibits a large range of δ60Ni values (from 0.02 ± 0.10 ‰ to 0.20 ± 0.05 ‰, n=7), explained by the diversity of Ni-bearing phases, and the average of δ60Nifeeding materials was found equal to 0.08 ± 0.08‰ (2SD, n=7). Both δ60Ni values of fly ash (δ60Ni = 0.07 ± 0.07‰, n=10) and final FeNi produced (0.05 ± 0.02 ‰, n=2) were not significantly different from the feeding materials ones. These values are consistent with the very high production yield of the factories. However, smelting slags present the heaviest δ60Ni values of all the smelter samples, with δ60Ni ranging from 0.11 ± 0.05 ‰ to 0.27 ± 0.05 ‰ (n=8). Soils were also collected near and far from the Niquelândia metallurgical plant, to evaluate the potential of Ni isotopes for tracing the natural vs anthropogenic Ni in soils. The Ni isotopic composition of the non-impacted topsoils developed on ultramafic rocks ranges from -0.26 ± 0.09 ‰ to -0.04 ± 0.05 ‰ (n=20). On the contrary, the Ni isotopic composition of the non-ultramafic topsoils, collected close to the plant, exhibit a large variation of δ60Ni, ranging from -0.19 ± 0.13 ‰ up to 0.10 ± 0.05 ‰ (n=4). This slight but significant enrichment in heavy isotopes highlight the potential impact of smelting activity in the surrounding area, as well as the potential of Ni isotopes for discerning anthropogenic samples (heavier δ60Ni values) from natural ones (lighter δ60Ni values). However, given the global range of published δ60Ni values (from -1.03 to 2.5 ‰) and more particularly those associated to natural weathering of ultramafic rocks (from -0.61 to 0.32‰), the use of Ni isotopes for tracing environmental contamination from smelters will remain challenging.
Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site.
Graphical abstract
Highlights
► Smelting and refining Ni ore laterites induce slight Ni isotope fractionation. ► δ60Ni values of anthropogenic Ni product fall within the range of terrestrial samples. ► Smelting slags are enriched in Ni heavy isotopes with respect to soils samples. ► Use of Ni isotopes for tracing environmental contamination remains challenging.
Nickel is an important metal in modern infrastructure and technology, with major uses in the production of stainless steel (60% of the global primary Ni consumption, Nickel Institute, 2013) and alloys and other application such as electroplating or production of rechargeable batteries (Mudd, 2010). It is therefore of major economic importance. The worldwide increasing demand of metals for economic purpose induces intense mining that
The δ60Ni of soils ranges from -0.26 ± 0.09 ‰ to 0.11 ± 0.10 ‰ (n=24; Tab. 3). Soils
developed on UM rocks in Barro Alto and Niquelândia show δ60Ni values ranging from -0.26
± 0.09 ‰ to 0.11 ± 0.10 ‰ (n=15) and from -0.09 ± 0.05 ‰ to -0.04 ± 0.05 ‰ (n=5),
respectively (Fig. 3). The non-UM topsoils (NQ_S5 to 8), collected at various distance from
the Niquelândia metallurgical plant (from 0.1 to 20 km away), exhibits δ60Ni values ranging
from -0.19 ± 0.13 ‰ to 0.10 ± 0.05‰ (n=4). The δ60Ni values of non-UM topsoils show a
general increase as the collection site gets closer to the metallurgical plant.
4. Discussion
4.1. Fractionation of Ni isotopes during smelting and refining
According to Moore (2012), the Barro Alto plant is designed to produce FeNi by processing
2.4 Mt/y of dry ore, at a Ni concentration close to 1.6 wt %. Nickel-containing material
feeding the process after drying, crushing and homogenizing, exhibits a large range of δ60Ni
values (0.02 to 0.20 ‰, with an average of 0.08 ± 0.08 ‰, n=7). This feeding material results
of the mixture of saprolitic and lateritic materials but also of the fly ash. Its bulk composition
is therefore variable in term of Ni-bearing constituents, like clay minerals and Fe-oxides
(Ratié et al., 2015). Such Ni-bearing minerals are present in both saprolitic and lateritic units
of the exploited weathering profiles, whose Ni isotopic compositions vary from -0.61 to +
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0.32 ‰ and 0.00 to + 0.13 ‰, respectively (Ratié et al., 2015). The mixing in different
proportions of those types of materials to match the optimal conditions of ore processing can
explain the variable δ60Ni values of the feeding material used in the process (Fig. 4).
In the electric arc furnace, nickel oxide is reduced to produce Ni metal in FeNi alloy, the final
Ni product. Ferronickel has an intermediate Ni isotopic signature (δ60Ni=0.04 - 0.07 ‰) that
falls within the range of the Ni-containing feeding material. This absence of fractionation is
consistent with the process efficiency, estimated at 88 ± 10%.
The unrecovered Ni is lost in the fly ash, that can contain up to 3 wt % Ni, and in the smelting
and refining slags ([Ni] = 1-6 g/kg). However, fly ash is reprocessed since the construction of
the new Barro Alto plant and one can consider that only Ni contained in the slags is
definitively lost from the process. Fly ash δ60Ni values (δ60Ni=0.01 - 0.20 ‰, with an average
of 0.07 ± 0.07 ‰, n=10) are similar to that of the feeding material. Assuming that fly ash are
the main contributors to Ni atmospheric emissions from the smelter, the disseminated Ni
cannot be isotopically distinguished from the feeding material. These results show that the
calcination step is not inducing Ni isotope fractionation.
This trend also observed for the Cu isotopes in emission of a metal aerosol plume in the
atmosphere around one of the major Pb-Zn refinery (Mattielli et al., 2006). In contrast, studies
about Zn and Cd fractionation during smelting and refining, have shown an enrichment in
light isotopes in the fly ash (Mattielli et al., 2009, Cloquet et al., 2006; Sonke et al., 2008;
Bigalke et al., 2010), significantly different from isotopic value of the feeding material.
While Zn and Cd present low boiling points (907 °C and 767 °C, respectively), the Cu and Ni
ones are very high (2,562 °C and 2,913°C, respectively). Whereas evaporation process has
been shown by Wombacher et al. (2004) to induce isotopic fractionation, in the case of Cd,
the temperature of the Ni ore calcination step (900 °C) is not high enough to induce Ni
evaporation and isotopic fractionation at this step.
The fraction of Ni not recovered as ferronickel ends up in the smelting and refining slags,
either in the glassy or mineral matrix, or as metallic droplets (Solar et al., 2009). The smelting
slags are isotopically heavier (δ60Ni=0.11 to 0.27 ‰, with an average of 0.18 ± 0.05 ‰, n=8)
than FeNi (0.04-0.07 ‰), highlighting that Ni fractionation occurs during the reduction of
Ni IIO. Similar heavy isotopic enrichment has been observed in smelting and refining Zn
smelting slags (Sonke et al., 2008; Shiel et al., 2010; Bigalke et al., 2010), and Pb-Zn
smelting slags, for Cd isotopes (Cloquet et al., 2006; Gao et al., 2008). Assuming that the
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reduction slags represent the last fraction of molted Ni during the reduction process, the
corresponding fractionation factor (αslag-feeding material) can be calculated close to 1.0001. In spite
of being very low comparatively to the fractionation factors proposed by Sonke et al. 2008
(1.0002-1.0004) or Sivry et al. 2008 (1.00013-1.00062) in the case of Zn, this value highlights
the significant Ni isotopic fractionation induced by the metallurgical process in the reduction
slags.
4.2. Ultramafic soil vs non-UM soils
The chemical composition of the soil samples collected on the UM complexes of Barro Alto
and Niquelândia is consistent with previous published data (Reeves et al., 2007; Garnier et al.,
2009 ; Van der Ent et al., 2015). The four topsoils sampled on the non-UM complex
“Anápolis-Itauçu” can be clearly distinguished from the topsoils developed on UM rocks, due
to their low Ni content (from 0.3 to 1.9 g/kg) and their low Mg/Al ratio (Fig. 5).
The δ60Ni values of the soils developed on UM rocks collected in Barro Alto and Niquelândia
range from -0.26 ± 0.09 ‰ to 0.11 ± 0.10 ‰, and are consistent with the few δ60Ni values
reported so far for soils developed on UM rocks (Estrade et al., 2015; Ratié et al., 2015). δ60Ni
values of the non-UM topsoils exhibit also a large variation in Ni isotopic composition (δ60Ni
= -0.17 to 0.10 ‰, n=4) but are not significantly different from topsoils developed on UM
rocks. However, in the samples NQ_S8, the high value of Ni and Mg content combined to the
heavy δ60Ni value can be reasonably assumed to be linked to anthropogenic contamination
4.3. Implications for tracing Ni contamination
Values of δ60Ni published so far for terrestrial samples are compiled together with our
data in the figure 6. The published δ60Ni values for “natural” terrestrial samples range from -
1.03 ‰ to 2.50 ‰ (see citations in figure caption). The δ60Ni values reported in this study for
anthropogenic samples (0.01 to 0.27 ‰), fall within the range of terrestrial sample signatures.
The potential anthropogenic impact of the smelter can be induced by both particulate and
dissolved metals released from smelting sub-product. Isotopes have already proved that they
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can be used as tracer of such contaminations (e.g. Bigalke et al., 2010; Sivry et al., 2008;
Sonke et al., 2008; Cloquet et al., 2006; Wen et al., 2015; Chrastny et al., 2015). In the case of
the Niquelândia and Barro Alto smelters, and considering the errors bars (2SD), FeNi and fly
ash δ60Ni values cannot be considered significantly different. The isotopic compositions of
contamination source issued from pyrometallurgical process are not significantly different
from isotopic composition of terrestrial samples observed in the soils and in the UM complex.
This feature emphasizes that the use of Ni isotopes for tracing environmental contamination
from smelters still remains challenging. However, the isotopic composition of dissolved Ni of
the settling pond water, i.e. in contact with fly ash for a long period, is heavier than the Ni
isotopic composition of fly ash. Moreover, it is also heavier than the dissolved Ni of the water
collected in the UM massif. This would imply that the release of Ni from anthropogenic
material, such as fly ash, can be distinguished from naturally dissolved Ni. Therefore, the Ni
isotopic signature in the dissolved phase could be promising to track an eventual
contamination of surface and groundwater. Further experiments are needed to evaluate the
impact of the waste storage on their isotopic signature. In that way, preliminary leaching
experiments have been performed on fly ash and slag at acid pH (supplementary data 1) in
order to evaluate the impact of H+-promoted weathering on the δ60Ni signature. The first
results seem to show that the preferential dissolution of some of the Ni-bearing phases such as
Ni-bearing serpentine-like phases, Ni-glass and olivine leads to the release of heavy Ni.
5. Conclusion
The present study on two metallurgical plants has shown that part of smelting and refining
activities induce a fractionation of Ni isotopes with a range of δ60Ni values from 0.01 ± 0.05
‰ (fly ash) to 0.27 ± 0.05 ‰ (smelting slags), consistent with the mass dependent law. Fly
ash δ60Ni values are similar to that of feeding material, showing that the calcination step is not
inducing a Ni fractionation. Ferronickel, i.e. the final Ni product, has an intermediate Ni
isotopic signature that falls within the range of the feeding material, which is consistent with
the very high Ni recovery yield of the entire process.
The enrichment in heavier isotopes in smelting slags, probably due to redox process in the
electric furnace, highlights the potential impact of smelting activity in the surrounding area.
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Moreover, dissolved Ni from the settling ponds, where fly ash is stored, appears to be
enriched in heavy isotopes compared to pristine water. These results highlight the potential
value of Ni isotopes to distinguish anthropogenic Ni (heavier δ60Ni signatures) from natural
one (lighter δ60Ni values). However, the whole range of δ60Ni values published so far for
terrestrial samples is larger than the range of δ60Ni values of metallurgical by-product. This
feature emphasizes that the use of Ni isotopes for tracing environmental contamination from
smelters still remains challenging.
Acknowledgements
This work was financially supported by the French Ministry of National Education and
Research (G. Ratié PhD grant), National French Program EC2CO from INSU, CNRS, and a
Marie Curie International Research Staff Exchange Scheme Fellowship within the 7th
European Community Framework Programme (NIDYFICS, n°318123). This work also
benefited from the Ciencia sem Fronteiras program (C. Quantin). The authors wish to thank
Anglo American for access to their field facilities, and the staff for help during sampling, as
well as O. Rouxel (IFREMER, France) for the Ni double spike preparation.
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Table 1: Total contents of elements and Ni isotopic composition (δ60
Ni, δ62
Ni) of mixed Ores (Ore), Fly ash (F), Smelting Slags (SS, a old,
b recent), White
Refining Slags (WRS), Black Refining Slags (BRS), FeNi.
Sample Name Ca Mg Fe Mn Ni δ
60Ni 2SD δ
62Ni 2SD
g/kg ‰
Niq
ue
lân
dia
Ore1 2.54 92.3 165 2.69 16.9 0.03 0.05 0.06 0.10
Ore2 1.79 97.3 148 2.42 23.2 0.03 0.05 0.06 0.09
Ore3 3.89 81.4 135 2.56 20.1 0.20 0.05 0.40 0.10
Ore4 2.70 103 151 2.89 17.6 0.05 0.05 0.06 0.12
F1 3.33 88.7 157 3.23 22.7 0.01 0.05 0.02 0.10
F2 2.85 90.6 193 3.24 27.0 0.03 0.06 0.06 0.12
F3 3.48 110 200 3.53 27.6 0.01 0.05 0.03 0.10
F4 3.30 73.5 209 2.9 24.6 0.06 0.08 0.11 0.15
F5 3.41 70.5 245 3.99 23.6 0.03 0.05 0.05 0.10
F6 1.90 92.4 178 5.13 22.9 0.15 0.06 0.28 0.14
F7 2.60 17.3 365 4.63 8.30 0.01 0.05 0.02 0.10
F8 5.10 20.9 311 3.48 7.80 0.05 0.05 0.10 0.10
F9 2.77 93.0 285 3.84 18.9 0.20 0.05 0.38 0.10
SS1a 12.83 55.1 131 2.39 1.81 0.11 0.05 0.21 0.10
SS2a 5.71 188 117 3.17 1.90 0.16 0.05 0.31 0.10
SS3b 2.68 161 111 2.86 1.17 0.20 0.05 0.39 0.10
SS4b 3.97 179 143 3.44 1.59 0.27 0.05 0.52 0.14
SS5a 2.20 155 122 2.89 0.98 0.17 0.11 0.31 0.25
FeNi1 1.86 <LD 661 <LD 338 0.04 0.05 0.08 0.10
Ba
rro
Alt
o
Ore5 1.93 95.9 118 2.62 18.5 0.02 0.10 0.05 0.20
Ore6 3.45 110 164 3.15 18.1 0.17 0.13 0.33 0.26
Ore7 1.60 71.4 178 3.02 22.2 0.04 0.06 0.07 0.13
F10 4.88 123 265 3.86 30.9 0.10 0.12 0.20 0.23
SS6b 4.19 187 68.8 2.95 1.64 0.18 0.05 0.35 0.10
SS7b 2.90 162 106 2.60 1.14 0.22 0.05 0.44 0.10
SS8b 3.42 153 124 2.89 2.03 0.14 0.05 0.26 0.10
WRS 69.5 143 71.2 1.97 2.28 0.14 0.06 0.27 0.13
BRS 149 102 179 1.59 6.28 0.03 0.06 0.06 0.12
FeNi2 0.30 <LD 689 <LD 311 0.07 0.06 0.13 0.13
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(δ60
Ni, δ62
Ni) of the water samples collected in surface of the Barro Alto massif (BA_W) and in
NQ_S8 5YR 3/3 non UM Niq (0.1km of plant) 1.17 0.56 7.22 84.91 122.70 1.38 1.92
0.10 0.05 0.20 0.09
NQ_S5 2.5YR 4/6 non UM Niq (1.8 km of plant) 0.19 0.70 4.59 88.12 153.43 2.01 0.30
0.03 0.05 0.07 0.10
NQ_S6 2.5YR 3/5 non UM Niq (5 km of plant) 0.27 0.05 0.89 96.93 147.07 1.37 0.50
0.07 0.05 0.14 0.04
NQ_S7 2.5YR 3/2 non UM Niq (20 km of plant) 1.89 0.72 0.71 108.89 200.17 1.70 0.57
-0.19 0.13 -0.37 0.25
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Figure Captions
Figure 1: Schematic view of the FeNi smelting and refining lines with the Ni content (adapted from Crundwell et al., 2011). Figure 2: δ60Ni versus δ62Ni plot showing all the solid samples analyzed in this study (n=54) and the regression line (black line) relative to the equilibrium mass-dependent fractionation line (red line). The slope of the regression line (0.5134) is identical to that of the theoretical equilibrium fractionation law (0.5164) within uncertainty. Figure 3: Average values of δ60Ni and δ62Ni for all samples (Ore: Feeding materiel (n=7), F:Fly ash (n=10, SS: Smelting Slag (n=10), RS: Refined slag (n=2) and FeNi (n=2), UM soil (n= 20), non UM soil (n=4).The error bar corresponds to SD. Figure 4: Comparison of saprolitic and lateritic ore samples (modified from Ratié et al., 2015) with feeding material introduced in the metallurgical process and soil samples (this study). Figure 5: Plot δ60Ni values vs molar ratio Mg/Al for the UM soils and non-UM soils. Figure 6: Variations in Ni isotopic composition (‰) of published terrestrial samples (Cameron et al., 2009; Gall et al., 2012; Gueguen et al., 2013; Cameron & Vance, 2014 ; Porter et al., 2014 ; Estrade et al., 2015; Ratié et al., 2015, Ventura et al., 2015) and by-product metallurgical samples from our study (Fly ash, slag, FeNi and waste water).
To understand Ni isotope systematics during proton-promoted dissolution of Ni-
bearing metallurgical materials, pH-stat leaching tests have been performed on slag and fly
ash samples to simulate the release of Ni under acidic conditions. Slag and fly ash samples
(SS3 and F2, respectively) were subjected to batch leaching at pH 3 using the approach based
on pH-static leaching experiment to European standard CEN/TS 14997 (CEN, 2006). The
leaching was carried out in duplicate at 20°C. A mass of 1g of solid was placed in 10 mL of
MilliQ+ deionized water and HNO3 was added to adjust the pH to the value of 3. The slag
sample was coarse-grained with 73% of particles between 0.5 and 2 mm, while fly ash was
fine-grained (75% particles smaller than 1 mm). Suspensions were continuously stirred for 48
hours, and leachates were filtered through 0.45 µm membrane and analyzed for Ni
concentration using either ICP-OES (Thermo Scientific iCAP 6500) or ICP-MS (Thermo
Scientific X-series II). Nickel isotopes were measured in extracts and residual materials using
the procedure described in the main text of the article.
Evolution of Ni isotopic composition during leaching experiment
Leaching experiments on fly ash sample (sample F2) released around 23% of total Ni initially
present in the sample, whereas only 6% of the total Ni was released from smelting slag
(sample SS3). The δ60Ni values of the corresponding leachates (δ60Ni = 0.12 ‰ and 0.26 ‰
for F2 and SS3 leachates, respectively, SI Fig. 1) are heavier than the solid residues, whose
isotopic signatures are δ60Ni = -0.07 ‰ and 0.07 ‰ for F2 and SS3 residues, respectively.
Nickel isotopic fractionation during proton-promoted weathering
Leaching tests were performed to understand the consequences of acid weathering on Ni
release (Ettler et al., 2015, submitted, SI for review only) and we report the associated
potential fractionation. The Ni leached from both slag and fly ash samples is isotopically
heavier than the residual Ni. The ∆leachate-residue for the both (F2 and SS3) is to 0.19 ‰
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60Niresidue). This feature can be explained by the preferential dissolution of
isotopically distinct phases during the incongruent dissolution of the samples, observed by
Ettler et al. (submitted). These results can be compared to those obtained for the stagnant
water collected in settling ponds in Niquelândia, where fly ash is stored for more than 20
years (δ60Ni from 1.63 to 1.81 ‰).
The Ni-bearing phases weathered are mainly responsible of the δ60Ni heavier value in the
leachates. In the slags, the small amount of Ni released (6%) can be explained by the
mineralogical composition dominated by amorphous phases, olivine, pyroxene and FeNi
particles (Ettler et al., 2015, submitted). Amorphous phases are expected to be weathered
more easily than the crystalline minerals; therefore the heavier pool observed in leachate
would originate from Ni scavenged in the amorphous phases.
Fly ash presents a mineralogical composition more heterogeneous than slags, with the
presence of magnesioferrite, talc, serpentine, amphibole, hematite, smectite, chlorite (Ettler et
al., 2015, submitted). These minerals are typical of saprolitic and lateritic ores (Gleeson et al.,
2004; Butt and Cluzel, 2013), which present a large variability in δ60Ni values (from -0.61‰
to 0.30‰; Gall et al., 2013; Ratié et al., 2015). The preferential dissolution of some of the Ni-
bearing phases such as Ni-bearing serpentine-like phases, Ni-glass and olivine (Ettler et al.,
2015, submitted) may explain the large proportion of Ni released (23%). The isotopic
composition of the leachate could indicate that these Ni-bearing phases are isotopically
heavier than the residual phases. This could be confirmed by the measurements of Ni isotopes
at a high spatial resolution, i.e. at the Ni-bearing phase scale.
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SI Figure 1: δ60Ni values for bulk samples (diamond) F2 (open symbols) and SS3 (closed symbols), their leaching fraction (square) and their residue (triangle) after controlled weathering.